I have decided to humanly euthanize this topic, the suffering on both threads is painful to watch.
Could you just call, say this dealer, https://www.brookmanstamps.com/2017-18US.pdf
and ask? Yours would be a very quick and precise question.
I just got off the phone minutes ago after speaking to a very pleasant and helpful employee who grabbed a 2021 Brookman and checked the "SO" listings and she reports there is no listing for SO-107A nor SO107B. She was wondering if you planned to contact their Brookman Editor and I advised I had no idea, I was just getting the answer to the listing question.
During the call I did mention the most recent copy in the APS Library is 2017. Perhaps they will send a free copy since they publish the Brookman.